Fluid flow within sediments in the San Diego Trough, off the coast of California, has evolved over the past 5 million years. Early on, hot water and gas spewed from active, bounding faults, driven by pressure from overlying sedimentary layers. But now hydrocarbons, including the greenhouse gas methane, seep slowly from the seafloor, driven by buoyancy [Boles et al., 2004].

The methane seepage in the San Diego Trough appeared to taper off starting in 2013, but now it appears to be active again. Changes in hydrostatic pressure due to sea level are not thought to have significantly changed over the past 2–3 years. Could recent earthquake activity have reactivated the methane seeps?

This was one of the questions on the minds of a group of early-career scientists as they took to the sea. From 1 to 17 December 2016, 21 graduate students and postdoctoral researchers participated in a marine geology and geophysics expedition on board the R/V Sikuliaq as it transited between Honolulu, Hawaii, and San Diego, Calif.

The at-sea scientific and leadership training experience, called the Chief Scientist Training Cruise (CSTC), was organized by the University-National Oceanographic Laboratory System (UNOLS) and was funded by the National Science Foundation. The yearly CSTCs aim to expose and prepare early-career scientists to take on leadership roles in planning, funding, and executing international collaborative seagoing expeditions.

Below is a progress report from one of those subgroups, whose members designed one leg of the expedition. This subgroup, informed by prior observations of methane seeps and faulting activity in the California Borderlands and the Santa Monica Basin [Maloney et al., 2015], made plans to map and observe these features in higher resolution.

The R/V Sikuliaq nears its destination after its transit from Honolulu, Hawaii, across the Pacific Ocean to San Diego, Calif. Credit: Jennifer Brizzolara

Preparing to Come Aboard

The CSTC participants got to work before even stepping on board the ship. During 2 days on land, they worked in subgroup sessions, hosted at the Department of Geology and Geophysics, University of Hawai‘i at Mānoa, to present competing proposals of feasible science objectives, appropriate target locations, and suitable methodologies that maximized the overall use of the 2-week transit.

The participants gained experience in designing and testing scientific hypotheses, manipulating the ship’s onboard technologies, and adhering to limitations imposed by the vessel’s capabilities. This scientific decision-making process was accompanied by lessons in managing real-world constraints of seagoing research, like unpredictable weather and team members’ scheduling.

As a part of the team’s preparation, they reviewed what scientists knew about the methane seeps in the San Diego Trough and decided how best to use their cruise time to help answer several puzzling questions.

Methane’s Role

Methane seepage appears to start and stop over periods of a few years, but questions remain about this pulsating behavior and how it is related to the fluid pressure in the area at any given time [Grupe et al., 2015; Maloney et al., 2015]. Previous studies have demonstrated that any variable affecting hydrostatic pressure (i.e., tides and sea level changes) could influence the stability of methane seepage [Boles et al., 2001].

Although most of the methane released from the seafloor dissolves before reaching the atmosphere, methane released in vigorous, episodic bursts (like the methane burst in the 2015 video below) could travel farther up the water column. If scientists are to make accurate estimates of seep mobilization and the amount of gas released into the ocean and atmosphere, they must understand the processes that control the dynamics of seeps.

Ongoing research is investigating the large-scale processes by which this gas can affect ocean chemistry and greenhouse gas concentrations. At present, however, no one knows how significantly this source contributes to the global methane budget and how that could be altered under a future climate change regime [Kessler, 2014].

The extent of hydrate deposits and the associated methane seepage activity may be connected to seismic activity.Active faulting and slumping in offshore environments could alter the dynamics of methane seepage activity [Paullet al., 2008; Yelisettiet al., 2014]. Studies of recent earthquakes off the coast of Southern California suggest that at least some of the faults in this region are, indeed, active [Hauksson et al., 2014].

Bottom-simulating reflectors, structures with acoustic signatures that mimic the seafloor’s signal and that are commonly associated with methane hydrates, have previously been observed within the California Borderland and the Santa Monica Basin [Torres et al., 2002]. Previous research has shown that fluid movement is actively associated with fault rupturing [Eichbul and Boles, 2000]. Thus, the extent of these hydrate deposits and the associated methane seepage activity may be connected to seismic activity.

Participants in one of the cruise projects, led by Jacob Beam, prepare to deploy the gravity corer to take sediment cores from the abyssal Pacific as part of a study of biological cycling of iron in marine sediments. Credit: Jennifer Brizzolara

Measuring the Methane

The CSTC participants collected acoustic reflection data using a subbottom profiler. They also gathered multibeam bathymetry and water column data using multibeam sonar with adjustable frequencies to identify active faults and associated methane hydrates (Figure 1).

The survey team revisited the previously surveyed Del Mar Seep located in the San Diego Trough offshore of Del Mar, Calif. Surveyors using multibeam bathymetry, subbottom profiler data, and video from a remotely operated vehicle (ROV) in 2012 were the first to observe this seep’s activity [Maloney et al., 2015]. A return survey in 2013 did not detect bubble activity [Grupe et al., 2015], suggesting that the seep was not active at that time.

The team aboard the R/V Sikuliaq during its visit on 16 December 2016 observed that the Del Mar Seep became active again.The team aboard the R/V Sikuliaq during its visit on 16 December 2016 observed that the seep became active again. Data from the subbottom profiler helped to determine the exact subsurface location of the methane seep on the basis of disruption in sedimentary layers (Figure 2). The team observed methane seeps covering a 300-meter distance along their scan at a depth of 1,020 meters (Figures 1 and 2).

Fig. 1. Location of the Del Mar Seep (32°N 54.22′, 117°W 46.92′). The inset shows the location of the methane seep relative to San Diego, Calif.; vertical exaggeration is 9 times. The solid black line shows the location of the subbottom data profile (Figure 2), corresponding to trace numbers 1550 to 1870.

Fig. 2. Subbottom acoustic cross section of the mound associated with the seep (see Figure 1 for the location of the scan line). TWT = two-way travel time of the sonar waves in seconds.

The seeps appear to be associated with a small mound feature on the seafloor. The shallow subsurface sedimentary layers are shifted by about 7–10 meters on either side of the seep location (Figure 2). Figure 3 shows the location where the team obtained a cross section of the water column multibeam data. Bubbles associated with the seep are visible in the water column (Figure 4).

Fig. 3. Multibeam bathymetry of the mound associated with the seep: The grid resolution is 5 × 6 meters, and the vertical exaggeration is 6 times. The dashed pink line represents the location of the water column cross section shown in Figure 4.Fig. 4. Water column imagery of the seep from the EM302 sonar. The seep trace identified shows methane rising up through the water column. The bottom trace identified represents the depth of the seafloor derived from multibeam bathymetry.

Valuable Training, Real Research Results

Ongoing questions about how methane seeps start and stop and what happens to the methane once it escapes the seafloor sediments will continue to drive scientific research for years to come. However, the results of this expedition highlight the impact of training early-career scientists and the importance of providing them with adequate tools and resources that teach them to plan and execute shipboard work in marine investigations. With the contributions to the community from newer, better-prepared generations of marine scientists, ocean exploration will remain at the forefront of critical scientific discoveries.

This year’s UNOLS CSTC ran from 26 September to 2 October, and the participants are hard at work processing the data they collected. Additional information about the UNOLS CSTC, a list of all 2016 participants, and a summary of the 2016 expedition can be found on the UNOLS website.

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